(a) Field of Invention
In many applications where fluid is employed as a power or energy medium, such as oil in a force applying or resisting hydraulic piston cylinder assembly, the inherent compressibility of the medium can create a serious disadvantage or limitation in the use of the device. Of course different mediums have different coefficients of compressibility and while in certain applications the design of the device can allow for the compressibility factor in others it presents a serious disadvantage resulting in curtailed use and/or expensive and inordinate auxiliary compensation equipment. For the purpose of explaining the present invention and not as a limitation of its application the well known problem of hydraulic actuators employed in strip or plate (strip) rolling mills to control the roll gap has been selected.
Many modern day rolling mills incorporate quick acting hydraulic piston cylinder assemblies (actuators) to control the gap between the work rolls and hence the gauge of the rolled product produced by the mill. Due to the fact that the actuator is usually arranged in the direct line of force of the rolling force to resist the force the amount or degree of the compressibility of the oil results in a "gauge error" since the compressibility allows the roll gap to be increased by the amount of the compressibility.
Because the gauge tolerance of rolled strip is generally very critical present mill designs have had to adhere to two additional design criteria, namely to keep the stroke of the actuator as small as possible and/or providing a control system to reposition the moveable element of the actuator to compensate for the element's movement due to the compressibility of the oil under the rolling force. Both of these criteria in many types of mills represent serious, if not prohibitive, limitations. Moreover, certain mill designs require long stroke actuators which in the past have necessitated some type of control system to compensate for the compressibility of the oil.
Such present day control systems employ some form of a position detector to detect fluid compression induced movement of the moveable element of the actuator which in turn requires the employment of an electrohydraulic servovalve unit. The systems in question usually operate under pressures of 4000 PSI in which the rolling force can range between 4,000,000 to 12,000,000 pounds for reduction mills, which means that considerable dynamic fluid energy and response time is inherently involved in effecting a correction for the compression of the fluid, more about which will be discussed later. The dynamics of this situation can result in the production of considerable off gauge rolled strip depending on the particular speed of the mill involved.
(b) Background Printed Information
Representative of prior publications discussing the concern of actuator compliance in rolling mill automatic gauge control systems (AGC Systems) may be found in U.S. Pat. Nos. 3,427,839 and 4,102,171 and in articles appearing in the "Iron and Steel Engineer" entitled Design and application of hydraulic gap control systems by Paul Huzyak et. al. - August, 1984, page 13 and Dynamic characteristics of automatic gauge control system with hydraulic actuators by Vladimir B. Ginzburg - January, 1984, page 57.